This invention relates to a photosensitive material which is useful for a fine working in the manufacture of a semiconductor element, a TFT (thin film transistor), a photo-disk, etc.
In the manufacture of electronic devices such as an LSI, a fine working technique utilizing a photo-lithography has been employed. In this fine working technique, a resist film is first formed by coating a resist solution onto a semiconductor substrate for instance. The resist film thus formed is then exposed to light through a prescribed mask pattern, and subsequently subjected to various treatments including an alkali-development, thereby forming a resist pattern. This resist pattern is then utilized as an etch resistant mask in a dry etch process, and the exposed portions of the substrate are etched away to form a pattern of fine line or window thereby obtaining a desired pattern. Finally, the resist pattern remaining on the substrate is removed through ashing.
Therefore, the resist to be employed in this process is generally required to have a high dry etch resistance. In view of this requirement, a resist comprising an aromatic compound has been extensively employed. For example, many kinds of resists comprising an alkali-soluble novolak resin as a base resin have been developed up to date.
In view of a trend in recent years to further increase the integration of semiconductor elements such as seen in an LSI, the aforementioned fine working technique in recent years has made it possible to realize a fine pattern of the order of sub-half micron. It is expected that this trend to further increase the integration of semiconductor elements becomes more prominent in future. In order to meet such a trend, the use of light source of shorter wavelength in photolithography is now being studied. For example, a method for forming a finer resist pattern through the use of ArF excimer laser (193 nm in wavelength) or the quintuple harmonic of YAG laser (218 nm in wavelength) has been tried at present.
However, the aforementioned conventional resist comprising a novolak resin as a base resin generally exhibits a high light absorption to the aforementioned light of short wavelength at the benzene nucleus of the novolak resin. Therefore, it is impossible, because of the high light absorption, to allow the exposure light to reach deep enough to an interface between the resist film and the substrate, thus making it very difficult to form, with high sensitivity, a fine pattern which is excellent in shape of pattern and in precision.
As one of the methods of forming a pattern, a method called PCM (portable comformable mask) method has been set forth by B. J. Lin in Proc. SPIE. 174, 114. According to this patterning method, a resist pattern is formed as an upper layer, and an underlying layer is collectively exposed using the resist pattern as a mask. One example employing this PCM method is reported by A. W. McCullough et al in Proc. SPIE. 631, 316, 1986. According to this document, PMGI (polydimethyl glutarimide) was employed as an underlying material and collectively exposed and patterned using as a mask an upper layer formed of novolak naphthoquinone diazide.
According to the aforementioned PCM method, even an underlying layer can be patterned by way of an exposure/development process in the transcription of pattern, so that the PCM method is advantageous in the respect that the generation of dust called scum as seen in the case of transcribing a pattern to an underlying layer by means of etching can be suppressed. Beside this PCM method, there have been proposed various kinds of PCM method. However, according to the conventional PCM methods, a photosensitive material employed for the underlying layer is, in most cases, relatively low in dry etch resistance as compared with the ordinary phenol resin, thus making it difficult to put these conventional PCM methods into practical use. In the case of the aforementioned method proposed by A. W. McCullough et al, novolak naphthoquinone diazide which is usually employed in a lithography using a mercury lamp is employed as a photosensitive material for an upper layer, while PMGI which is usually employed in a lithography using KrF excimer laser beam which wavelength is shorter than that of the mercury lamp is employed as a photosensitive material for a lower layer. Accordingly, the exposure light of relatively longer wavelength is employed for forming a pattern in the upper layer, thus rendering the method disadvantageous in terms of forming an optically fine pattern. Moreover, since there is actually a problem of an interlayer mixing in the lamination of a plurality of photosensitive layers, it has been very difficult to form a fine pattern.
As mentioned above, since the resist comprising a novolak resin as a base resin is insufficient in transparency to the light of short wavelength in spite of its excellent dry etch resistance and alkali-developing properties, there has been a strong demand for the development of a new resist which is suited for use in a photolithography where the ArF excimer laser or the quintuple harmonics of YAG laser is employed.
In view of this, a resist where an alicyclic compound is employed in place of an aromatic compound is recently attracting many attentions. For example, a resist comprising a polymer having an adamantine skeleton as a base resin thereof is proposed in Japanese Patent Unexamined Publication Hei/4-39665 as being excellent in dry etch resistance and in transparency to a light of short wavelength. This patent publication also discloses an example where a compound having an adamantine skeleton is copolymerized with an acrylic compound having a carboxylic group to provide a copolymer having an alkali-solubility, thereby making it possible to form a resist pattern with an alkaline developing solution.
However, various problems will be encountered in a process of forming a resist pattern through an alkaline development of a resist comprising an alicyclic compound, because the alkali-solubility of an alicyclic structure such as the adamantine skeleton differs greatly from that of carboxylic group. For example, the solubility and removal of a given region of a resist film may become non-uniform at the occasion of development, thereby inviting not only the deterioration of resolution, but also the generation of cracks or surface roughening of a resist film portion which should have been kept unremoved due to a partial solution of the resist film portion. Moreover, an alkali solution may penetrate into an interface between the resist film and the substrate, thereby causing a peeling of the resist pattern, thus failing to assure a sufficient adhesion of the resist. Additionally, a phase separation between the alicyclic structure portion and the carboxylic group portion may be readily proceeded, thus making it difficult to prepare a homogeneous resist solution and to obtain a resist solution having a satisfactory coating property.
There has been also developed, as a highly sensitive resist, a chemical amplification type resist utilizing a catalytic effect of an acid. However, this chemical amplification type resist is vulnerable to the influence during a time period between the exposure and the PEB. In particular, due to an influence by the amine contained in the air atmosphere, the shape of resist pattern to be obtained may be deteriorated, so that under some circumstances, it may result in a peeling of the resist pattern from the substrate.
Furthermore, as mentioned above, according to the conventional PCM method, the photosensitive material for the upper layer is constituted by a material to be employed in a lithography employing a light whose wavelength is longer than that to be employed in a lithography for treating the underlying layer, thus rendering the method disadvantageous in terms of forming an optically fine pattern.